Parameters

Property Value

Type: TValueThe value associated with the specified key. If the specified key is not found, a get operation throws a KeyNotFoundException, and a set operation creates a new element with the specified key.

This property provides the ability to access a specific element in the collection by using the following C# syntax: myCollection[key] (myCollection(key) in Visual Basic).

You can also use the Item property to add new elements by setting the value of a key that does not exist in the Dictionary<TKey, TValue>. When you set the property value, if the key is in the Dictionary<TKey, TValue>, the value associated with that key is replaced by the assigned value. If the key is not in the Dictionary<TKey, TValue>, the key and value are added to the dictionary. In contrast, the Add method does not modify existing elements.

A key cannot be null, but a value can be, if the value type TValue is a reference type.

The C# language uses the this keyword to define the indexers instead of implementing the Item property. Visual Basic implements Item as a default property, which provides the same indexing functionality.

Getting or setting the value of this property approaches an O(1) operation.

The following code example uses the Item property (the indexer in C#) to retrieve values, demonstrating that a KeyNotFoundException is thrown when a requested key is not present, and showing that the value associated with a key can be replaced.

The example also shows how to use the TryGetValue method as a more efficient way to retrieve values if a program often must try key values that are not in the dictionary.

This code example is part of a larger example provided for the Dictionary<TKey, TValue> class. openWith is the name of the Dictionary used in this example.

// The Item property is another name for the indexer, so you // can omit its name when accessing elements.
Console.WriteLine("For key = \"rtf\", value = {0}.",
openWith["rtf"]);
// The indexer can be used to change the value associated // with a key.
openWith["rtf"] = "winword.exe";
Console.WriteLine("For key = \"rtf\", value = {0}.",
openWith["rtf"]);
// If a key does not exist, setting the indexer for that key // adds a new key/value pair.
openWith["doc"] = "winword.exe";
...
// The indexer throws an exception if the requested key is // not in the dictionary. try
{
Console.WriteLine("For key = \"tif\", value = {0}.",
openWith["tif"]);
}
catch (KeyNotFoundException)
{
Console.WriteLine("Key = \"tif\" is not found.");
}
...
// When a program often has to try keys that turn out not to // be in the dictionary, TryGetValue can be a more efficient // way to retrieve values. string value = "";
if (openWith.TryGetValue("tif", out value))
{
Console.WriteLine("For key = \"tif\", value = {0}.", value);
}
else
{
Console.WriteLine("Key = \"tif\" is not found.");
}